For words that only have three letters, there is absolutely no 4th letter. As an example with the word “insert”, there are no more letters afterwards and so have to be the word "add". The word “addict” is during the BIP39 wordlist however you might have used “addi” for “addict”. https://thesocialdelight.com/story4405079/top-latest-five-bip39-seed-phrase-mnemonics-generator-urban-news